Opinion poll

An opinion poll, often simply referred to as a survey or a poll, is a human research survey of public opinion from a particular sample. Opinion polls are usually designed to represent the opinions of a population by conducting a series of questions and then extrapolating generalities in ratio or within confidence intervals. A person who conducts polls is referred to as a pollster.

History

The first known example of an opinion poll was a tally of voter preferences reported by the Raleigh Star and North Carolina State Gazette and the Wilmington American Watchman and Delaware Advertiser prior to the 1824 presidential election, showing Andrew Jackson leading John Quincy Adams by 335 votes to 169 in the contest for the United States presidency. Since Jackson won the popular vote in that state and the national popular vote, such straw votes gradually became more popular, but they remained local, usually citywide phenomena.
In 1916, The Literary Digest embarked on a national survey and correctly predicted Woodrow Wilson's election as president. Mailing out millions of postcards and simply counting the returns, The Literary Digest also correctly predicted the victories of Warren Harding in 1920, Calvin Coolidge in 1924, Herbert Hoover in 1928, and Franklin Roosevelt in 1932.
Then, in 1936, its survey of 2.3 million voters suggested that Alf Landon would win the presidential election, but Roosevelt was instead re-elected by a landslide. George Gallup's research found that the error was mainly caused by participation bias; those who favored Landon were more enthusiastic about returning their postcards. Furthermore, the postcards were sent to a target audience who were more affluent than the American population as a whole, and therefore more likely to have Republican sympathies. At the same time, Gallup, Archibald Crossley and Elmo Roper conducted surveys that were far smaller but more scientifically based, and all three managed to correctly predict the result. The Literary Digest soon went out of business, while polling started to take off. Roper went on to correctly predict the two subsequent reelections of President Franklin D. Roosevelt. Louis Harris had been in the field of public opinion since 1947 when he joined the Elmo Roper firm, then later became partner.
In September 1938, Jean Stoetzel, after having met Gallup, created IFOP, the Institut Français d'Opinion Publique, as the first European survey institute in Paris. Stoetzel started political polls in summer 1939 with the question "Why die for Danzig?", looking for popular support or dissent with this question asked by appeasement politician and future collaborationist Marcel Déat.
Gallup launched a subsidiary in the United Kingdom that was almost alone in correctly predicting Labour's victory in the 1945 general election: virtually all other commentators had expected a victory for the Conservative Party, led by wartime leader Winston Churchill. The Allied occupation powers helped to create survey institutes in all of the Western occupation zones of Germany in 1947 and 1948 to better steer denazification. By the 1950s, various types of polling had spread to most democracies.
Viewed from a long-term perspective, advertising had come under heavy pressure in the early 1930s. The Great Depression forced businesses to drastically cut back on their advertising spending. Layoffs and reductions were common at all agencies. The New Deal furthermore aggressively promoted consumerism, and minimized the value of advertising. Historian Jackson Lears argues that "By the late 1930s, though, corporate advertisers had begun a successful counterattack against their critics." They rehabilitated the concept of consumer sovereignty by inventing scientific public opinion polls, and making it the centerpiece of their own market research, as well as the key to understanding politics. George Gallup, the vice president of Young and Rubicam, and numerous other advertising experts, led the way. Moving into the 1940s, the industry played a leading role in the ideological mobilization of the American people in fighting the Nazis and the Japanese in World War II. As part of that effort, they redefined the "American Way of Life" in terms of a commitment to free enterprise. "Advertisers", Lears concludes, "played a crucial hegemonic role in creating the consumer culture that dominated post-World War II American society."

Statistics

If we ask a yes-no question of a sample of people selected randomly from a large population, then the proportion of the sample that respond "yes" will be close to the true proportion, , of the whole population who would have said "yes" had all of them been asked.
The distribution of the proportion of 'yes' answers follows the binomial distribution. A binomial distribution converges to a normal distribution if the size of the sample approaches infinity according to the central limit theorem.
In practice the binomial distribution is approximated by a normal distribution when and where is the sample size. The larger is the sample, the better is the approximation.
Suppose that people were sampled, and a share of them responded "yes". This sample proportion can be used instead of, which is unknown, to compute the sample mean, variance and standard deviation.
The sample mean is:.
The sample variance is:.
The sample standard deviation is:.

Example

Assume that we conduct a poll in which people are asked whether they support candidate A. We sample 1000 people of which 650 respond "yes". In this case
and. Therefore, we can approximate the binomial distribution by using the normal distribution.
As a rule of thumb, we want our poll result to be accurate within the 5% significance level. Therefore, we will compute the confidence interval:
The sample mean is:.
The sample variance is:.
The sample standard deviation is:.
We shall use the formula to create a confidence interval with 95% confidence level:

where is the population mean and is the z-score for 95% confidence level.
or:
That is, we are 95% confident that the true population mean,, is between 620.44 and 679.55.
Remembering that , we can say that or is 0.65 with a margin of error equal to 3%.

Sample sizes

The number of people needed to create a valid sample depends on the population size and required margin of error.
We shall use Cochran's formula: ,
where is the z-score for a confidence level of and is the required margin of error.
Note that the function is maximized at, therefore, before starting sampling we will use to determine the sample size.
For example, assume that we want 95% confidence level and 5% margin of error:
.
Note that the required sample size is affected by the confidence level and margin of error.
If we want 99% confidence interval we have to sample 664 people, and, alternatively, if we want a margin of error of 2% we will have to sample 2401 people.
For a finite population, when the sample is a large proportion of population, we modify the formula:
where N is the size of the entire population. Note that as N approaches infinity, the two formulas coincide, meaning the consideration of population size can only reduce the required sample size needed for a valid sample.
In the above example, if the entire population is 600 then we have to sample only 285 people.

Sample and polling methods

Opinion polls for many years were maintained through telecommunications or in person-to-person contact. Methods and techniques vary, though they are widely accepted in most areas. Over the years, technological innovations have also influenced survey methods such as the availability of electronic clipboards and Internet based polling.
Opinion polling developed into popular applications through popular thought, although response rates for some surveys declined. Also, the following has also led to differentiating results: Some polling organizations, such as Angus Reid Public Opinion, YouGov and Zogby use Internet surveys, where a sample is drawn from a large panel of volunteers, and the results are weighted to reflect the demographics of the population of interest. In contrast, popular web polls draw on whoever wishes to participate.
Statistical learning methods have been proposed in order to exploit social media content for modelling and predicting voting intention polls.

Benchmark polls

A benchmark poll is generally the first poll taken in a campaign. It is often taken before a candidate announces their bid for office, but sometimes it happens immediately following that announcement after they have had some opportunity to raise funds. This is generally a short and simple survey of likely voters. Benchmark polling often relies on timing, which can be a significant problem if a poll is conducted too early for anyone to know about the potential candidate. A benchmark poll needs to be undertaken when voters are starting to learn more about the possible candidate running for office.
A benchmark poll serves a number of purposes for a campaign. First, it gives the candidate a picture of where they stand with the electorate before any campaigning takes place. If the poll is done prior to announcing for office the candidate may use the poll to decide whether or not they should even run for office. Secondly, it shows them where their weaknesses and strengths are in two main areas. The first is the electorate. A benchmark poll shows them what types of voters they are sure to win, those they are sure to lose, and everyone in-between these two extremes. This lets the campaign know which voters are persuadable so they can spend their limited resources in the most effective manner. Second, it can give them an idea of what messages, ideas, or slogans are the strongest with the electorate.